Mission's end A pair of robotic space probes circling the Moon to reveal what is inside will plunge into the lunar surface tomorrow, a planned - albeit dramatic - finale

The Gravity Recovery and Interior Laboratory, or GRAIL, mission will come to an end at 9:28 am AEDT when the twin spacecraft crash into a mountain near the Moon's north pole after nearly a year in lunar orbit.

The duo have been formation-flying to map the Moon's gravity, an innovative technique that has revealed a lunar crust that is thinner and far more deeply fractured than scientists expected and an extensive underground system of lava-filled cracks, the first direct evidence that the moon expanded after it was formed.

The information applies not just to the Moon, but to the other solid bodies in the inner solar system, including Earth and Mars.

Seeing the extent of the damage from impacting comets and asteroids, for example, makes it easier to visualise how water on the surface of ancient Mars might have made it way inside the planet, where it might still exist today.

"There's a lot of questions about where did the water that we think was on the surface of Mars go. Well, if a planetary crust is that fractured these fractures provide a pathway deep inside the planet. It's very easy to envision now how a possible ocean on the surface could have found its way deep into the crust of the planet," says GRAIL lead scientist Maria Zuber, with the Massachusetts Institute of Technology.

Measuring Moon's ebb and flow

The GRAIL spacecraft, nicknamed Ebb and Flow, completed their primary mapping mission in May, flying about 50 kilometres above the lunar surface. By precisely and continuously measuring the distance between the two probes, scientists were able to map the Moon's gravity, revealing its interior structure.

The distance changed slightly as the leading spacecraft and then the following one sped up or slowed down as they flew over denser or less-dense regions of the moon in response to the gravitational tugging.

After several months, the pair's orbit was lowered to about 20 kilometres above the surface for more detailed mapping. Now, about out of manoeuvring fuel, the spacecraft are down to about 12 kilometres while scientists make a last map of the youngest crater on the Moon.

"We have achieved everything that we could have possibly hoped for," says Zuber. "In my wildest dreams, I could not have imagined that this mission would have gone any better than it has."

Silent end

The crash site was selected to avoid the possibility that the probes would crash into any artefacts left behind by the Apollo and other lunar missions.

The spacecraft will hit the surface at about 6000 kilometres per hour. No pictures are expected because the region will be in darkness at the time of impact.

"We are not expecting a big crash or a big explosion. These are two small spacecraft. They are apartment-sized washer- and dryer-sized spacecraft with empty fuel tanks. So we're not expecting a flash that is visible from Earth," says Zuber.

However, a sister spacecraft circling the moon, NASA's Lunar Reconnaissance Orbiter, will attempt to make observations, Zuber adds.

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